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In general, it has been extremely challenging to impart fire resistance into polyester resins and fibers without detracting from other properties such as processability, ability to melt spin fibers, and mechanical properties. In addition, small molecule flame retardants can leach from the polyester and contaminate the environment, and as a result, certain flame retardants have been banned in several countries. Thus, there is a recognized need to provide fire resistance to polyester fibers in an environmentally friendly manner without detracting from melt processability, strength, modulus, dyeing and heat-setting characteristics as compared to the unmodified polyester.
Flame retardant polyesters must be resistant to degradation by residual acidic groups in the polyester, exhibit long-term dimensional stability, have good dyeing characteristics in the final fiber, and exhibit good mechanical properties. The requirements for flame retarding polyesters are stringent in part because of the high processing temperatures and sensitivity of the polyesters' physical properties such as melt viscosity to the addition of flame retardants. For example, small molecule flame retardants often increase the melt viscosity of the polyester which reduces the melt processability of the polyester making it more difficult to make fibers by melt spinning. These challenges combined with environmental regulations for toxicity and mitigation of leaching of the flame retardant into the environment over time have made it extremely difficult to meet all of these requirements.